Nowadays, technique for manufacturing a thin film transistor (hereinafter, referred to as a TFT) over a substrate is drastically progressing, and the application and development thereof to an active matrix display device are being advanced. In particular, a TFT using a polycrystalline semiconductor film has higher electron field-effect mobility (also referred to as mobility) than that of a TFT using a conventional non-single crystal semiconductor film; therefore, high speed operation can be conducted. Hence, the control of a pixel which is conventionally conducted in a driver circuit provided outside a substrate has been tried to conduct in a driver circuit formed over the same substrate as a pixel.
Meanwhile, as for a substrate used for a semiconductor device, a glass substrate is considered to be more promising substrate than a single crystal semiconductor substrate in terms of cost. The glass substrate is inferior in heat resistance and easy to be deformed by heat; therefore, in the case of forming a TFT using a polycrystalline semiconductor film over the glass substrate, laser annealing is employed for crystallizing the semiconductor film to avoid heat deformation of the glass substrate.
Laser annealing has a feature of being able to reduce treatment time drastically compared with an annealing method utilizing radiation heating and conduction heating, giving little heat damage to a substrate by heating a semiconductor substrate or a semiconductor film selectively or locally, and the like.
The term “laser annealing” herein used indicates technique for recrystallizing an amorphous layer or a damaged layer formed over a semiconductor substrate or a semiconductor film and technique for crystallizing an amorphous semiconductor film formed over a substrate. In addition, technique which is applied to surface modification or planarization of a semiconductor substrate or a semiconductor film is also included.
A laser oscillator used for laser annealing is classified broadly into two kinds of pulsed oscillation and continuous oscillation by the oscillation method. In recent years, it has been found that a grain size of a crystal formed in a semiconductor film is large in crystallization of a semiconductor film in the case of using a continuous wave laser oscillator (also referred to as a CW laser) such as Ar laser or YVO4 laser compared with a pulsed laser oscillator such as excimer laser. In the case of laser beam irradiation, the laser beam is transformed using an optical system so as to have a linear shape in an irradiated surface, and emitted by moving the emitting position of the laser beam comparatively with respect to the irradiated surface. This method is industrially superior because of having high productivity.
The term “linear shape” herein used does not mean “line” in the strict sense but means a rectangle or an ellipse with high aspect ratio (for example, with aspect ratio of 10 or more (preferably 100 to 10000)).
Thus, when the grain size of a crystal in a semiconductor film becomes large, the number of grain boundaries which go into a TFT channel region formed using this semiconductor film is decreased and mobility is increased, and accordingly, it can be utilized for the development of a device with higher performance. Therefore, a continuous wave laser oscillator attracts attention.
In this specification, a region in which a large grain size crystal is formed in a semiconductor film is referred to as a large grain size region, and a region in which a small grain size crystal is formed in a semiconductor film is referred to as a poorly crystalline region.
Laser beam with a wavelength of from a visible region to an ultraviolet region is frequently used for laser annealing of a semiconductor film. This is because absorption efficiency of energy to a semiconductor film is favorable. However, the wavelength of a fundamental wave oscillated by solid CW (continuous wave) laser which is generally used is a wavelength region in the range of from red to near infrared. Therefore, a method for being converted into harmonic wave with a wavelength of a visible region or less using a nonlinear optical element is employed. In general, a visible light is generated by converting a fundamental wave into a harmonic wave using a nonlinear optical element, and the light is used for annealing of a semiconductor film.
For example, a laser beam oscillated from a CW laser which provides 10 W at 532 nm is transformed into a linear beam of 30 μm in a major axis direction and approximately 10 μm in a minor axis direction. In the case where this linear beam is made to scan in a minor axis direction and a semiconductor film is crystallized, the width of a large grain size region obtained at one time scanning is approximately 200 μm. Therefore, in order to conduct laser crystallization to the whole surface of the substrate, it is necessary to conduct laser annealing by shifting a position scanned by linear beam to a long side by the width of the large grain size region obtained at one time scanning of linear beam. Further, if annealing is conducted by emitting a laser light of a plurality of lasers to different regions simultaneously over a substrate, efficiency can be more enhanced.
By a crystallization step of a semiconductor film by a CW laser, a poorly crystalline region is formed separately from a large grain size region. The poorly crystalline region is formed in a portion where energy is lowered in both edges of a linear beam. Therefore, the poorly crystalline region is formed in both edges interposing the large grain size region. When a semiconductor element is formed in a portion including a poorly crystalline region, an object which is inferior in characteristics to a semiconductor element formed in a large grain size region is formed. A general manufacturing method of a TFT is as follows: a semiconductor film formed over a substrate is set to be an irradiated surface, and a marker or the like is formed on this semiconductor film, and then laser irradiation is conducted so that a poorly crystalline region is not formed in a portion where a semiconductor element of the semiconductor film is formed to form a large grain size region. Therefore, the position which is to be irradiated with laser is required to be precisely determined. To accomplish this, a method for controlling an irradiated position by providing a marker for an irradiated surface, which is to be a standard, detecting this marker by a CCD camera or the like, and conducting image processing using a computer is employed. This marker is used for determining a light-exposure position in a photolithography step after a laser crystallization step.